[SPARK-6391][DOCS] Document Tachyon compatibility.

Adds a section in the RDD persistence section of the programming-guide docs detailing Spark-Tachyon version compatibility as discussed in [[SPARK-6391]](https://issues.apache.org/jira/browse/SPARK-6391).

Author: Calvin Jia <jia.calvin@gmail.com>

Closes #6382 from calvinjia/spark-6391 and squashes the following commits:

113e863 [Calvin Jia] Move compatibility info to the offheap storage level section.
7942dc5 [Calvin Jia] Add a section in the programming-guide docs for Tachyon compatibility.